Institute of Electrical and Electronics Engineers (IEEE) 802.11 is a set of physical and media access control (MAC) specifications for implementing wireless local area network (WLAN) communications. These specifications provide the basis for wireless network products using the Wi-Fi brand managed and defined by the Wi-Fi Alliance. The specifications define the use of the 2.400-2.500 GHz as well as the 4.915-5.825 GHz bands. These spectrum bands are commonly referred to as the 2.4 GHz and 5 GHz bands. Each spectrum is subdivided into channels with a center frequency and bandwidth. The 2.4 GHz band is divided into 14 channels spaced 5 MHz apart, though some countries regulate the availability of these channels. The 5 GHz band is more heavily regulated than the 2.4 GHz band and the spacing of channels varies across the spectrum with a minimum of a 5 MHz spacing dependent on the regulations of the respective country or territory.
IEEE 802.11 defines a data frame exchange process that enables the devices, referred to as stations and access points, to negotiate the timing of the exchange of data between devices over the various shared channels of the 2.4 GHz and 5 GHz bands. As used herein an access point (AP) is a particular type of station, however, station (STA) is generally used to refer to non-AP stations for clarity. The data frame exchanges could be performed with single-antenna transmission or multiple-antenna transmission (also referred to as multiple-input multiple-output or MIMO). That is each of the stations or access points in the WLAN may include and utilize a single antenna or multiple antennas. In the case of multiple-antenna or MIMO transmission, multiple spatial streams (SS) are sent within the same frame from one station or access point, which usually is called a beamformer (BFer), to another station or access point, which is usually called a beamformee (BFee), and this type of transmission is called beamforming (BF), or MIMO. BF and MIMO transmissions are usually enhanced by some initial frame exchanges so that the BFer knows about the MIMO channel condition of the BFee. This initial exchange of frames before the actual data frame exchange is called a sounding procedure. The frames that might be used in a sounding procedure are the high throughput (HT) and very high throughput (VHT) null data packet (NDP) frame, VHT MIMO Compressed Beamforming Report frame, VHT NDP Announcement (NDPA) frame, and VHT Beamforming Report Poll frame. Each of these frames may have various fields and subfields such as: VHT MIMO Control, VHT Compressed Beamforming Report, MU Exclusive Beamforming Report, Sounding Dialog Token, STA Info, and similar fields that are utilized for exchanging information relevant to beamforming.